Spring driven injection devices are known and used to assist patients and medical professionals with the injection of medications held in conventional plunger driven syringes. Such injection devices typically comprise a housing for containing the syringe to which a needle is fixed, and a drive member attached to a relatively strong compression spring for driving the plunger into the syringe body to eject medication through the needle and into a patient.
By way of example, International patent application publication number WO94/21316 describes an injection device substantially as illustrated in FIG. 1. This device is intended to completely eject all medication from a syringe, i.e. it has no mechanism for adjusting the ejected dose. The injection device comprises a distal housing part 1 into which is screwed a proximal housing part 2 (where the terms “distal” and “proximal” are used here and in the following text to identify respectively locations distant from and close to a patient's skin when the device is in use). An adjustable needle shield 3 is further screwed into a proximal end of the proximal housing part 2. A housing sleeve 4 is slideably mounted on top of the distal housing part 1. As is shown in FIG. 1, the injection device comprises a trigger 5 which projects through a window 6 provided in the sleeve 4.
FIG. 2 illustrates the prior art device of FIG. 1 in cross-section. The device is shown with a syringe 7 located within the housing, where the syringe comprises a syringe body 8 having a pair of radially extending “wings” 9 at one end and a needle coupling 10 formed at the other end. A needle 11 is shown attached to the syringe body using the needle coupling 10. The syringe also comprises a plunger 12 having a rubber bung 13 attached to one end and an enlarged end face or plate 14 formed at the other end to assist in pushing the plunger through the syringe body. The syringe is supported within the proximal housing part 2 by a short support tube 19 that sits coaxially within the proximal housing part 2. A collar 23 is provided around the distal end of the support tube 19, and provides an end stop for a relatively light coil spring 24. The other end of the coil spring 24 is stopped by a locking tube 25 which is screwed to an interior surface of the proximal housing part 1. The locking tube 25 also fixes the support tube 19 into the proximal housing part 2 such that this support tube is fixed relative to the proximal housing part.
As shown in FIG. 2, a drive member 15 is slideably located within the distal housing part 1. The drive member is generally cylindrical having a generally square cross-section. Located within the interior space of the drive member 15 is a relatively strong compression drive spring 16. The drive spring is supported on a cylindrical support shaft 17 which is in turn fixably attached to the distal housing part 1 via an elongate pin 18.
FIG. 2 illustrates the device in the primed or cocked position, where the drive member 15 has been pushed into the distal housing part 1 in the distal direction, thereby compressing the drive spring 16 along the support shaft 17. FIG. 2 shows the trigger 5 pivotally coupled to the distal housing part 1 and partially projecting through the window 6 formed in the sleeve 4. The trigger comprises at an outer distal end thereof a button part 20 and, at an inner proximal part, a catch 21. In the position illustrated in FIG. 2, the catch 21 is engaged with a corresponding recess 22 formed in the drive member 15 such that the drive member is held by the trigger in the primed state. Although not shown in the figures, the trigger 5 is biased about its pivot axis such that the catch is pressed against the drive member 15 in the absence of any external force.
In order to release the drive member 15 from the primed state and thereby push the needle 11 into a patient's skin and inject medication from the syringe 7, a user holds the sleeve 4 in a fist-like grip and presses the needle shield 3 against his or her skin. This causes the sleeve 4 to slide over the distal housing part housing 1, closing the gap identified in FIG. 2 with the reference A. This relative movement of the sleeve and the distal housing part causes the window 6 to move wholly over the trigger 5 such that, when the user exerts pressure on the trigger button part 20 with his or her thumb, the trigger pivots, disengaging the catch 21 from the recess 22. Once the drive member 15 is released, the force exerted by the expanding drive spring 16 pushes the drive member 15 forward and into engagement with the end plate 14 of the syringe plunger 12.
At this stage in the operation of the device, the syringe body 8 is relatively free to move through the proximal housing part 2 such that the force exerted by the drive member 15 on the plunger end plate 14 causes the entire syringe 7 to move towards the distal end of the injection device. The wings 9 formed on the end of the syringe body 8 act on the support tube collar 23, causing the support tube 19 to also move towards the proximal end of the device, compressing the relatively light coil spring 24. Once this spring is fully compressed, further movement of the support tube 19 and therefore of the syringe body 8 is prevented. Of course, during this preliminary phase, the needle is projected beyond the end of the needle shield and into the patient's skin. The extent to which the needle projects beyond the end of the needle shield 3 can be adjusted by screwing the shield into and out of the proximal end of the proximal housing part 2.
As the driver member 15 continues to exert a significant force on the plunger 12 after further movement of the support tube 19 and the syringe body 8 has been prevented, the plunger 12 begins to move through the syringe body 8 thereby ejecting medication through the needle 11.
It is noted that the device illustrated in FIGS. 1 and 2 comprises a further, relatively light compression spring 26 that is connected between the distal housing part 1 and the sleeve 4. This spring 26 is relatively light, and allows the sleeve 4 to be returned to its original position once an injection has been completed and the user has both removed the needle shield 3 from contact with the skin and has released the pressure on the trigger button 20.
In order to assemble the device of FIGS. 1 and 2, the proximal and distal housing parts are separated, and a complete syringe including medication is dropped into the proximal housing part through the larger open end thereof so that it is supported within the support tube 19. With the housing parts still separated, the drive member 15 is pressed into the distal housing part 1 (using, for example, a cocking tool) until the catch 21 on the trigger 5 engages with the drive member recess 22 and locks the drive member in the cocked position. The device can then be assembled by screwing the proximal and distal housing parts together.
In order to take into account different tolerances within the injection device and the syringe, the injection device is configured such that, when the device is fully assembled, a small gap is present between the end plate 14 of the plunger 12 and the opposed end of the drive member 15. This gap is identified in FIG. 2 with the reference B. In the absence of such a gap, the drive member could come into contact with the end plate of the plunger during connection of the housing parts, causing some movement of the syringe through the lower housing towards the distal end of the device. It is also possible that some initial sticking of the syringe body could occur, resulting in the ejection of a small volume of medication from the syringe.
A potential problem observed in the prior art device such as that illustrated in FIGS. 1 and 2 arises from the relatively high force exerted by the drive member 15 on the syringe plunger 12. A high force is of course required in order to eject the medication through the needle and into the patient. However, the initial impact caused by the drive member 15 hitting the end plate 14 of the syringe plunger 12 may damage the plunger and or the drive member. For example, it is possible that the end of the plunger may fracture. It is also possible that the collar 23 formed on the support tube 19 and/or the syringe wings 9 may be damaged when the spring 24 is compressed to its full extent, i.e. immediately prior to injection of medication from the syringe.